Intrusion alarm system with turbulence compensation

ABSTRACT

Ultrasonic intrusion alarm system with oppositely phased voltage doublers and timing capacitors for distinguishing between the movements of an intruder and other movements such as air turbulence. An operational amplifier in the input stage provides common mode rejection; an integrator prevents actuation of the alarm by disturbances of short duration, and fail-safe means is included to prevent the system from being deflated by disabling it.

United States Patent Hankins et a].

[451 Jan. 25, 1972 [72] Inventors: Thomas C. Hankins; David Glen Barleen,

both of Oakland, Calif.

[73] Assignee: Systron-Donner Corporation, Concord,

Calif.

[22] Filed: June26, 1970 [21] Appl.No.: 50,233

3,293,631 12/1966 Premack ..340/258 2,769,972 11/1956 MacDonald ..340/258 A 2,655,645 10/1953 Bagno ....340/258 A X 2,714,205 7/1955 Grayson et al. .340/258 A UX 3,242,486 3/1966 Corbell .340/258 A X 3,418,649 12/1968 Williamson.... ..340/258 Primary Examiner lohn W, Caldwell Assistant Examiner-Glen R. Swann, lll Attorney-Flehr, Hohbach, Test, Albritton & Herbert V V a W V [57] 7 ABSTRACT [52] US. Cl. ..340/258 A, 340/261 Ultrasonic immsion alarm System with oppositely phased vol [51] Int. Cl. ..G08b 13/18 age doubles and i i imr for distinguishing between [58] Field of Search ..340/261, 258, 258 D, 258 A, the movements of an intruder and other movements such as 340/258 258 R air turbulence. An operational amplifier in the input stage provides common mode rejection; an integrator prevents actua- [56] References cued tion of the alarm by disturbances of short duration, and fail- UNITED STATES PATENTS safe means is included to prevent the system from being deflated by disabling it. 3,383,678 5/1968 Palmer ..340/258 X 3,081,433 3/1963 Bosch et al ..340/25s x 8 Claims, 3 Drawing Figures 19.2 KHz 05C.

FROM DET.

PMENIEDWZEWZ 3.638.210

FIG 3 INVENTORS THOMAS C. HANKINS DAVID G. BARLEEN ATTORNEYS INTRUSION ALARM SYSTEM WITH TURBULENCE COMPENSATION BACKGROUND OF THE INVENTION This invention pertains generally to intrusion detectors and more particularly to an ultrasonic system which includes means for distinguishing between the movements of an intruder and other movements such as air turbulence.

In one type of intrusion detector system heretofore provided, ultrasonic radiations are transmitted into a space such as a room. These radiations are reflected from the walls or other objects in the room to a receiver-where they are compared with the radiations as originally transmitted. In the absence of motion within the protected room, the received radiations will have the same frequency as the transmitted radiations. When an object moves within the room, the radiations reflected by the moving object arrive at the receiver with a different frequency than that of the transmitted radiations. The difference in frequency is primarily due to the Doppler effect and is dependent upon the rate of movement of the object. The difference in frequency is sensed by the receiver and utilized for actuating an alarm.

Intrusion detector systems of the foregoing type which have heretofore been provided have certain limitations. For example, events other than the movement of an intruder can produce changes in the frequency of the reflected radiations and thus tend to cause false alarms. These other events include air turbulence such as that which occurs when a heater is turned on in the protected room. In addition, noise and transient disturbances such as pipes knocking or a book falling from a shelf can cause false alarms.

Heretofore, certain attempts have been made to eliminate the false alarms due to air turbulence. The most common of these is based upon the observation that the signals due to air turbulence are generally substantially smaller in magnitude than the signals due to an intruder. Thus, the air turbulence signals have been discriminated against simply by reducing the sensitivity of the system to the point where they cannot actuate the alarm. This approach has the drawback of reducing the overall sensitivity and effectiveness of the system without entirely eliminating false tripping due to air turbulence.

Another prior attempt at eliminating false tripping due to air turbulence is described in U.S. Pat. No. 2,794,974, issued June 4, 1957, and U.S. Pat. No. 3,111,657, issued Nov. 19, i963. This approach is based upon the fact that the amplitude of the signals due to air turbulence drops off sharply above a frequency on the order of hertz whereas the signals due to an intruder have a substantially constant amplitude up to approximately 100 hertz. According to this approach, the turbulence and intruder signals are separated into two separate amplifier channels, one containing only frequencies on the order of 2S hertz or higher and the other containing only frequencies on the order of 5 hertz.

Thus, the high-frequency channel contains only the intruder signal, and the low-frequency channel contains the turbulence signal plus a portion of the intruder signal. The signal in the high-frequency channel is amplified, and the outputs of the channels are compared to produce a control signal for actuating an alarm. This alarm is energized only when the amplified output of the high-frequency channel "is greater than the unamplified output of the low-frequency channel, i.e., only when an intruder is present. In the absence of an intruder, the output of the high-frequency channel, even though amplified, is too small relative to the output of the low-frequency channel for the alann to be actuated. While this approach provides some improvement over the previous approach of merely reducing the sensitivity of the system, it is not entirely effective in distinguishing intruder signals from the turbulence signals. In addition, it requires two separate channels, an amplifier, and a pair of filters for separating the input signal into the two channels.

There is, therefore, a need for a new and improved intrusion alarm system which overcomes the foregoing and other problems encountered with the system heretofore provided.

SUMMARY AND OBJECTS OF THE INVENTION The intrusion alarm system of the present invention ineludes means for distinguishing signals due to an intruder from those due to air turbulence without reducing the overall efficiency of the system and without requiring two separate channels. According to the invention, the difference signal is passed through a steep-skirted band-pass filter having a center frequency on the order of 40 hertz and a 3 db. bandwidth on the order of 6 hertz. This filtered signal is applied to a pair of oppositely phased voltage doublers which produce positive and negative voltages having magnitudes dependent upon the duration and rate of disturbances in the filtered signal. These positive and negative voltages are combined to provide an output signal having an amplitude and polarity dependent upon the duration and rate of the disturbances. This output signal controls the actuation of an alarm. An integrator connected between the output of the voltage doublers and the alarm prevents false alarms to transients and other disturbances of very short duration.

It is in general an object of the present invention to provide a new and improved intrusion alarm system.

Another object of the invention is to provide a system of the above character which includes means for distinguishing signals due to movements of an intruder and from those due to air turbulence.

Another object of the invention is to provide a system of the above character which includes means for preventing false alarms due to transients and other disturbances of short duration.

Another object of the invention is to provide a system of the above character which includes means for cancelling the effects of common mode noise at the input of the system.

Another object is to provide a system of the above character which includes failsafe means for energizing the alarm in the event of an interruption of the radiated signals.

Additional objects and features of the invention will be apparent from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of one embodiment of an intrusion alarm system incorporating the present invention.

FIG. 2 is a schematic diagram of the input and mixer stages of the embodiment illustrated in FIG. 1.

- FIG. 3 is a schematic diagram of the filter, turbulence compensator, integrator, and relay driver stages of the embodiment illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As illustrated in FIG. I, the alarm system of the present invention includes an oscillator 10 connected for delivering energy at a predetermined frequency to a transducer or speaker 11 from which it is radiated into a space or room to be protected. A pickup device 12 is provided for receiving the radiated energy as it is reflected by objects in the room. The output of the pickup device is delivered to an amplifier l3 and then to a mixer 14 where it is combined with the signal from the oscillator 10 to produce a Doppler or difference signal having a frequency corresponding to the rate of movement of the objects in the room. A detector 16 removes the oscillator frequency from the output of the mixer, and this detected signal is passed through a band-pass filter 17 to a turbulence compensator 18. The output of the turbulence compensator is connected to the input of an integrator 19, and the output of the integrator is connected to a relay driver 21 which includes a relay for controlling the actuation of an alarm 22. The signal from the oscillator 10 is also applied to the relay driver 21 for actuating the alarm or producing an alarm condition, in the event that the operation of the oscillator is interrupted.

The oscillator 10 is of conventional design and is adapted for generating energy at a predetermined frequency,

preferably in the ultrasonic range. In the presently preferred embodiment, the oscillator operates at a frequency of 19.2 kilohertz. The transducer 11 can be of the type described in U.S. Pat. No. 3,287,693, issued Nov. 22, 1966 and assigned to the assignee of the present invention. The pickup device 12 is likewise of conventional design.

The amplifier 13 includes an operational amplifier 26 having an inverting input terminal 27, a noninverting input terminal 28, and an output terminal 29. The energy received by the pickup device 12 is applied to the inverting and noninverting input terminals through a coupling transformer 31 and a network comprising capacitors 32, 33, 34 and resistors 36, 37. Thus, the energy received by the pickup device is applied between the inverting and noninverting input terminals to the operational amplifier and appears in amplified form at the output 29. However, common mode noise and other signals which are applied in phase to the inverting and noninverting input terminals are cancelled and produce no output at the terminal 29. A feedback resistor 38 is connected between the output and inverting input terminals of the operational amplifier 26 to maintain the gain of the amplifier at a constant level.

The mixer 14 includes an operational amplifier 41 having an inverting input terminal 42, a noninverting input terminal 43, and an output terminal 44. The oscillator signal is applied to the inverting input terminal 42 through resistors 46, 47 and a capacitor 48. The output of the operational amplifier 26 is connected to the noninverting input terminal 43 through a RC filter network 51, a gain control 52, a capacitor 53, and a resistor 54. The oscillator signal and the amplified received signal are combined in the operational amplifier of the mixer to provide at the output terminal 44 a Doppler or difference signal having a frequency corresponding to the difference in frequency between the transmitted and received signals.

The detector 16 is of conventional design, such as a diode detector.

The band-pass filter 17 includes an operational amplifier 46 having low-pass and high-pass filters connected in a negative feedback loop between its output terminal 47 and inverting input terminal 48. The low-pass filter includes series resistors 49, 51 and a shunting capacitor 52 connected from the junction of the resistors to ground. The high-pass filter includes series capacitors 53, 54 and a shunting resistor 56 connected from the junction of the capacitors to ground. In the preferred embodiment, the filter 17 has band-pass centered at a frequency on the order of 40 hertz, with a 3 db. bandwidth on the order of 6 hertz. The resistors 49, 51 and capacitor 52 are chosen for feeding back signals below the desired center frequency, and the capacitor 53, 54 and resistor 56 are chosen for feeding back signals above this center frequency.

The output of the band-pass filter 17 is connected through a capacitor 57 and resistor 58 to an amplifier stage consisting of an operational amplifier 59. The gain of this amplifier is maintained at a constant level by a feedback resistor 61 connected between the output terminal 62 and inverting input terminal 63 of the operational amplifier.

The output of the operational amplifier 59 is connected to the input of the turbulence compensator 18 which includes a pair of oppositely phased voltage doublers 63, 64. The voltage doubler 63 includes a capacitor 66, a charging resistor 67, and diodes 68, 69. The diodes are connected in such manner that a positive voltage is produced between the cathode of the diode 68 and the anode of the diode 69. This voltage has a magnitude on the order of twice the peak-to-peak voltage at the output of the amplifier 59. The voltage doubler 64 includes a capacitor 71, a charging resistor 72, and diodes 73, 74. These diodes are connected in such manner that a negative voltage is produced between the anode of the diode 73 and the cathode of the diode 74. The anode of the diode 69 and cathode of the diode 74 are connected together to provide a common output terminal for the two voltage doublers.

Timing capacitors 76, 77 are connected across the outputs of the voltage doublers 63, 64 respectively. The capacitor 77 is chosen to have a value on the order of times the value of the capacitor 76, and consequently the output of the voltage doubler 63 changes more rapidly than does the output of the doubler 64. Thus, the capacitors 76, 77 in effect tune the voltage doublers so that high-frequency changes in the output of the amplifier 59 appear as positive voltages at the cathode of the diode 68 and low frequency changes appear as negative voltages at the anode of the diode 73.

The charging resistors 67, 72 are chosen to have different values of resistance. In the preferred embodiment, the resistor 67 has a resistance on the order of one-half the resistance of the resistor 72. Thus, when the voltages across the capacitors 76, 77 have risen to their maximum values, the magnitude of the positive output of the doubler 63 will be on the order of twice the magnitude of the negative output of the doubler 64.

The cathode of the diode 68 and the anode of the diode 73 are connected to a summing network consisting of resistors 78, 79, and 81. The resistor 81 has an adjustable tap or wiper 82 which provides means for selectably adjusting the relative components of the positive and negative voltages in the output of the turbulence compensator.

The integrator 19 includes an operational amplifier 86 having an integrating capacitor 87 connected between its output terminal 88 and inverting input terminal 89. The output of the turbulence compensator is applied to the inverting and noninverting input terminals of this operational amplifier. The wiper 82 of the summing resistor 81 is connected directly to the inverting input terminal 89, and the common output of the voltage doublers is connected to the noninverting input terminal through a diode 92. Means is provided maintaining the noninverting input terminal 91 at a reference voltage on the order of 0.2 volts. This means includes a source of +6 volts connected to the common output of the voltage doubler and to the cathode of the diode 92, together with a resistor connected between the noninverting input terminal and a source of +l2 volts. Thus, the noninverting input terminal of the integrator is maintained at a voltage on the order of 0.2 volts above the common output of the voltage doublers, and the integrator delays the actuation of the alarm until the voltage applied to the inverting input terminal remains above this value for a period determined by the value of the integrating capacitor 87.

The output of the integrator 19 is connected to the relay driver through a diode 93 and a resistor 94. The relay driver includes transistors 96, 97 and 98 connected for controlling the energization of a relay coil 99. The transistors 97, 98 are connected in series with the relay coil and are biased in such manner that they are normally conducting so that in the absence of an alarm condition the relay coil remains energized. The alarm means 22 is connected to the contacts of the relay in such manner that as long as the coil 99 remains energized, the alarm will not be actuated. The base of the transistor 96 is connected for receiving the signal from the integrator through the resistor 94, and the collector of this transistor is connected to the base of the transistor 98. A diode is connected in the emitter circuit of the transistor 96 to establish a reference level that must be overcome by the out put of the integrator in order to turn off the transistor 98 and deenergize the relay coil 99.

The transistor 96 constitutes a portion of a fail-safe circuit for deenergizing the relay coil and actuating the alarm in the event that the oscillator 10 is disabled or otherwise ceases to operate. This circuit includes an input terminal 101 to which the oscillator 10 is connected. The oscillator signal is applied to a detector circuit comprising diodes 102, 103, and the output of the detector is applied to the base of the transistor 97 through a resistor 104. A filter capacitor 106 is provided for smoothing the output of the detector. Thus, the detected oscillator signal produces a negative voltage at the base of the transistor 97, holding this transistor in its conducting condition.

Operation and use of the alarm system of the present invention can now be described briefly. Let it be assumed that the system has been installed in a room and that the transducer 11 is radiating energy at a frequency 19.2 kilohertz. In the absence of motion within the room, the signal picked up by the pickup device also has a frequency of 19.2 kilohertz. Since both of the signals applied to the mixer have the same frequency, the output of the mixer is zero, and there is no signal to cause actuation of the alarm.

When an intruder enters the room, the received signal differs in frequency from the transmitted signal by an amount corresponding to the rate of movement of the intruder. In this situation, the output of the mixer 14 is a signal having a frequency corresponding to the difference in frequency between the transmitted and received signals. Since the different parts of an intruders body normally move at different rates, the output of the mixer is actually a band of frequencies corresponding to the rates of movement of the different parts, rather than the single frequency. With an intruder walking at a normal rate, these frequencies are concentrated at approximately 40 hertz and are passed by the bandpassed filter 17.

Frequencies on the order of 40 hertz cause the voltage to rise faster across the smaller timing capacitor 76 than across the capacitor 77. Hence, the voltage applied to the inverting input terminal 89 of the integrating amplifier is positive in polarity and has an amplitude corresponding to the magnitude of the received signal. In this situation, the output of the integrating amplifier is negative in polarity and is applied to the base of the transistor 96 through the diode 93. When the voltage across the capacitor 87 increases, the base of the transistor 96 becomes more negative, the transistor conducts more heavily, and its collector becomes more positive. When the voltage across the capacitor 87 reaches the tripping level set by the diode 95, the voltage at the collector of the transistor 96 turns off the transistor 98, deenergizingthe coil 99, thereby causing an alarm condition.

When movement within the room is at a rate slower than the normal rate of movement of an intruder, the signal applied to the turbulence compensator 18 generally consists of slow rates of change. These constant slow movements produce substantially simultaneous positive and negative voltage rises across the timing capacitors 76 and 77, respectively. With the outputs of the voltage doublers substantially equal in magnitude and opposite in polarity, the input to the integrator is substantially zero, and the alarm is not actuated. Thus, air turbulence, which produces maximum energy in the very low frequency region cannot cause actuation of the alarm.

Even when a low-frequency movement of sufficient magnitude persists for a long period of time, the alarm will not be actuated. However, an intruder cannot defeat the system by crawling or otherwise moving at an unusually slow rate of speed, because various parts of his body move at different speeds and produce eratic movements of many frequencies. In this situation, the alarm is actuated because the smaller capacitor 76 will charge very rapidly and the larger capacitor 77 will not be able to counteract this rapid charging. This causes the positive output voltage of the voltage doubler 63 to exceed the negative output of the doubler 64 by an amount corresponding to the relative sizes of the charging resistors 67, 72 and the magnitude of the received signal.

A shock noise or transient disturbance has a high frequency and therefore produces a positive voltage at the output of the turbulence compensator. However, this voltage is of such short duration that the integrating capacitor 87 cannot be charged to the value necessary to turn off the transistor 98 and actuate the alarm. Thus, because of the action of the integrating capacitor, the system is immune to noises such as pipes knocking or books falling from shelves.

If the intruder were to disable the oscillator 10, this would remove the negative voltage from the base of the normally conducting transistor 97. The positive supply voltage +V would then be applied to the base of this transistor through the resistor 107, and the transistor would be turned ofi, deenergizing the relay coil 99 and causing an alarm condition.

It is apparent from the foregoing that a new and improved intrusion alarm system has been provided. This system includes means for distinguishing the movements of an intruder from other disturbances such as air turbulence and shock noises. in addition, it includes means for giving an alarm even though the intruder moves abnormally slowly or disables the oscillator. While only a presently preferred embodiment of the invention has been described herein, as will be apparent to those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.

We claim:

1. In an intrusion alarm system, means for radiating a signal at a predetermined frequency into a space, means for receiving the signal as it is reflected from objects within the space, the received signal having a frequency differing from that of the radiated signal by amounts corresponding to the rates of movement of said objects, means for combining the received and radiated signals to produce a difference signal having a frequency spectrum corresponding to differences in the frequencies of the radiated and received signals, band-pass filter means connected for filtering the difference signal and passing a frequency range centered in the higher portion of the frequency spectrum of the difference signal, turbulence compensation means comprising a pair of tuned voltage multipliers connected for receiving the signal from said filter means and producing voltages of opposite polarities having magnitudes corresponding to the amplitudes of the components at different frequencies in the output of said filter, and means for combining the outputs of said voltage multipliers to produce a signal corresponding to the components in the filter output.

2. An intrusion alarm system as in claim 1 wherein the means for combining the received and radiated signals includes a mixer comprising an operational amplifier having inverting and noninverting input terminals and an output terminal, the radiated and received signals being applied to the input terminals of said operational amplifier for producing the difference signal at said output terminal.

3. In an intrusion alarm system, means for radiating a signal at a predetermined frequency into a space, means for receiving the signal as it is reflected from objects within the space, the received signal having a frequency differing from that of the radiated signal by amounts corresponding to the rates of movement of said objects, means for combining the received and radiated signals to produce a difference signal having a frequency spectrum corresponding to differences in the frequencies of the radiated and received signals, band-pass filter means connected for filtering the difference signal and passing a frequency range centered in the higher portion of the frequency spectrum of the difference signal, turbulence compensation means connected for receiving the filtered signal and producing a compensated signal dependent upon the amplitude and duration of the filtered signal, and alarm means connected to be actuated in response to said compensated signal, said turbulence compensation means including first and second oppositely phased voltage doublers each driving its input from the output of said band-pass filter means and each including a timing capacitor connected at its output, said timing capacitors being of different values so that the oppositely polarized outputs of said voltage doublers rise at different rates corresponding to different frequencies in the filtered signal, and summing means connected to the outputs of said voltage doublers for producing a signal having a magnitude and polarity dependent upon the relative amplitudes of the different frequencies in the filtered signal.

4. An intrusion alarm system as in claim 3 wherein said voltage doublers include series connected input resistors of different values so that signals of long duration and having a magnitude above a predetennined level will produce a signal sufficient for actuating said alarm means.

5. An intrusion alarm system as in claim 3 together with integrator means connected for receiving the signal from said summation means and delaying activation of said alarm means until said signal remains above a predetermined level for a predetermined period of time.

6. An intrusion alarm system as in claim 1 wherein said alarm means is normally maintained in a deenergized condition by normally energized means connected in series with first and second normally conducting switching means, said first switching means being controlled by the signal from said compensator means and said second switching means being controlled by said radiated signal so that interruption of said radiated signal causes energization of said alarm means.

7. An intrusion alarm system as in claim 1 wherein the means for receiving the reflected signal includes an operational amplifier having inverting and noninverting input terminals across which the reflected signal is applied, said operational amplifier providing means for passing said reflected signal and rejecting signals which appear in phase on each of the input terminals.

8. In an intrusion detector system of the type including means for comparing the frequencies of radiated and reflected signals to produce an alarm signal having a frequency dependent on the difference in the frequencies of said radiated and reflected signals, said difference depending upon the rate of movement in a protected space, means for distinguishing between movements of an intruder and other movements such as air turbulence, said last-named means including a band-pass filter connected for filtering the alarm signal to pass a band of frequencies centered at a frequency on the order of 40 hertz, a pair of tuned voltage doublers connected for receiving the output of said band-pass filter and producing voltages of opposite polarities having magnitudes corresponding to the amplitudes of the components at different frequencies in the output of said filter, summing means connected to the outputs of said voltage doublers for producing a signal having a magnitude and polarity dependent upon the relative amplitudes of the components in the filter output. 

1. In an intrusion alarm system, means for radiating a signal at a predetermined frequency into a space, means for receiving the signal as it is reflected from objects within the space, the received signal having a frequency differing from that of the radiated signal by amounts corresponding to the rates of movement of said objects, means for combining the received and radiated signals to produce a difference signal having a frequency spectrum corresponding to differences in the frequencies of the radiated and received signals, bandpass filter means connected for filtering the difference signal and passing a frequency range centered in the higher portion of the frequency spectrum of the difference signal, turbulence compensation means comprising a pair of tuned voltage multipliers connected for receiving the signal from said filter means and producing voltages of opposite polarities having magnitudes corresponding to the amplitudes of the components at different frequencies in the output of said filter, and means for combining the outputs of said voltage multipliers to produce a signal corresponding to the components in the filter output.
 2. An intrusion alarm system as in claim 1 wherein the means for combining the received and radiated signals includes a mixer comprising an operational amplifier having inverting and noninverting input terminals and an output terminal, the radiated and received signals being applied to the input terminals of said operational amplifier for producing the difference signal at said output terminal.
 3. In an intrusion alarm system, means for radiating a signal at a predetermined frequency into a space, means for receiving the signal as it is reflected from objects within the space, the received signal having a frequency differing from that of the radiated signal by amounts corresponding to the rates of movement of said objects, means for combining the received and radiated signals to produce a difference signal having a frequency spectrum corresponding to differences in the frequencies of the radiated and received signals, bandpass filter means connected for filtering the difference signal and passing a frequency range centered in the higher portion of the frequency spectrum of the difference signal, turbulence compensation means connected for receiving the filtered signal and producing a compensated signal dependent upon the amplitude and duration of the filtered signal, and alarm means connected to be actuated in response to said compensated signal, said turbulence compensation means including first and second oppositely phased voltage doublers each driving its input from the output of said bandpass filter means and each including a timing capacitor connected at its output, said timing capacitors being of different values so that the oppositely polarized outputs of said voltage doublers rise at different rates corresponding to different frequencies in the filtered signal, and summing means connected to the outputs of said voltage doublers for producing a signal having a magnitude and polarity dependent upon the relative amplitudes of the different frequencies in the filtered signal.
 4. An intrusion alarm system as in claim 3 wherein said voltage doublers include series connected input resistors of different values so that signals of long duration and having a magnitude above a predetermined level will produce a signal sufficient for actuating said alarm means.
 5. An intrusion alarm system as in claim 3 together with integrator means connected for receiving the signal from said summation means and delaying activation of said alarm means until said signal remains above a predetermined level for a predetermined period of time.
 6. An intrusion alarm system as in claim 1 wherein said alarm means is normally maintained in a deenergized condition by normally energized means connected in series with first and second normally conducting switching means, said first switching means being controlled by the signal from said compensator means and said second switching means being controlled by said radiated signal so that interruption of said radiated signal causes energization of said alarm means.
 7. An intrusion alarm system as in claim 1 wherein the means for receiving the reflected signal includes an operational amplifier having inverting and noninverting input terminals across which the reflected signal is applied, said operational amplifier providing means for passing said reflected signal and rejecting signals which appear in phase on each of the input terminals.
 8. In an intrusion detector system of the type including means for comparing the frequencies of radiated and reflected signals to produce an alarm signal having a frequency dependent on the difference in the frequencies of said radiated and reflected signals, said difference depending upon the rate of movement in a protected space, means for distinguishing between movements of an intruder and other movements such as air turbulence, said last-named means including a bandpass filter connected for filtering the alarm signal to pass a band of frequencies centered at a frequency on the order of 40 hertz, a pair of tuned voltage doublers connected for receiving the output of said bandpass filter and producing voltages of opposite polarities having magnitudes corresponding to the amplitudes of the components at different frequencies in the output of said filter, summing means connected to the outputs of said voltage doublers for producing a signal having a magnitude and polarity dependent upon the relative amplitudes of the components in the filter output. 